The invention relates generally to the field of ignition spark timing circuits for internal combustion engines and more particularly to the field of electronic ignition spark timing circuits in which a predetermined advance angle versus engine speed characteristic is produced.
In an internal combustion engine which uses a spark to ignite a fuel and air mixture in a cylinder, the proper timing of the spark with respect to the compression cycle of the cylinder has been a continuing problem. This problem exists because there are several different variables which influence the desired spark timing required for the efficient operation of the internal combustion engine. The ignition spark timing is primarily a function of the speed of the engine and the load on the engine, the engine load commonly being sensed by the amount of vacuum pressure produced by the engine. The engine speed is commonly measured in terms of the angular rotation of the engine crankshaft and the term "rotational speed of the engine" as used in this specification refers to the engine speed.
Prior art spark timing circuits have generally mechanically created an advance angle versus speed variation by using centrifugal force created by the engine crankshaft rotation to physically displace a cam which controls the amount of engine spark advance. The terms advance and advance angle as used in this specification generally refer to the phase difference between the occurance of a cylinder ignition spark and a predetermined reference position of the cyliner piston with respect to its compression cycle. The piston movement is determined by the crankshaft rotation and the advance angle is commonly measured in degrees of crank-shaft rotation.
Similarly, prior art vacuum advance systems have required the physical displacement of a cam as a function of the engine manifold pressure, this cam being coupled to a mechanical breaker system which produces a predetermined amount of spark timing advance as a function of engine manifold pressure. These prior art mechanical systems are extremely complex, difficult to adjust for a specific desired advance angle characteristic and are costly since they include a large number of mechanical parts which must be manufactured to very tight tolerances. In addition, the use of a large number of mechanical parts results in the unreliability of the spark timing circuit due to the frictional wearing out of these mechanical parts and the inherent unreliability of a very complex system. Thus the prior art mechanical spark timing circuits are not only hard to adjust but also require frequent spark timing adjustments due to the mechanical wearing out of the parts.
Prior art electronic spark timing circuits have obviated many of the disadvantages of the mechanical spark timing circuits but have been unable to accurately and simply reproduce the advance angle versus speed characteristic which was produced by the prior art mechanical systems. Some prior electronic systems develop a D.C. control voltage, by integration techniques, which is proportional to engine speed. This control voltage is then used to breakdown a zener diode and thereby create a different advance angle vs speed characteristic for all speeds above a predetermined speed. However, these systems can not respond rapidly to changes in speed because the control voltage is produced by integration. In addition, most prior art electronic systems have not provided a single complete circuit which produces a combined desired advance angle versus speed characteristic with a desired advanced angle versus engine manifold pressure characteristic (vacuum advance).